The rate of which your character regenerates hit points, mana and movement points is determined by a number of factors. Your base abilities are important, as are your race and your age, your being hungry or thirsty and your position (standing, sitting, sleeping...)
Spoiler
The formula for mana regeneration is pretty close to:
Mana/min = (-5 + (Wil/2) + (race bonus) + (Max mana)/8 + (age/EQ/potion mana regen bonus)) * (fullness effect) * (resting effect) * (hunger effect)
- (race bonus) is 0 for human, 1 for half-elf, 2 for elf
- (age bonus) can be found in age (e.g., +3 at 3rd age and +6 at 4th age)
- (EQ bonus) can be found from identify (e.g., a golden ruby ring is +4).
- (Potion bonus) can also be found from identify (e.g., a softly glowing green cup starts out as +10)
- (Max mana) includes any bonus from eq (e.g., an archaic copper wristband is +10).
- (fullness effect) ranges from about 0.85 if empty to 1.17 if full.
- (resting effect) is 1.0 if standing, 1.25 if resting, 2.0 if sleeping.
- (hunger effect) is something like 0.5 if hungry or thirsty (not tested carefully, it might be worse).
So, for instance, a naked 3.5 age half-elf with 129 max mana while standing will average
(4 + 129/8 + 4.5(age)) = 24.5 mana/minute
when half full, and will take (129/24.5) minutes = 316 seconds to fully regen from frozen. This rate increases to 28.5 when full, and decreases to 20.5 when frozen.
If the same character wears a mana ring and sleeps, they will average
(4 + 129/8 + 4.5(age) + 4(ring)) * 2 = 57 mana/minute
and fully recover from frozen in (129/57)*60 = 136 seconds. Strangely, the (fullness effect) seems to be slightly stronger when sleeping; it ranges from 0.78 to 1.22 or so, or in this case 44 (frozen) to 69 (burning) mana/min.
Note that the formula implies that an archaic copper wristband (+10 max mana) or rubellite (+11 max mana) actually give more regen than a slim silvery wristband (+1 mana regen).